Process for evaporating liquids



UNITED STATES PATENT OFFICE PROCESS FOR EVAPORATING LIQUIDS Casimir Theiler, Basel, Switzerland, assignor to Society of Chemical Industry in Basle, Basel,

Switzerland Application July 29, 1933, Serial No. 682,891 In Switzerland August 12, 1932 7 Claims.

In evaporating systems it has hitherto been possible to increase the heat economy and in part also the efficiency of evaporation by means of vacuum, multi-stage effect and heat pumps. For the evaporation of liquids sensitive to temperature changes vacuum has been used, but this increased the expenditure of energy and the costs. Evaporators are also known, which, by the introduction of air enable to realize an evaporation temperature of water below 100 C. at atmospheric pressure. Such systems are, however, uneconomical as the air is introduced at a temperature considerably lower than that corresponding to the evaporation point, whereby an increased consumption of heat by the evaporator is necessary in order to raise the temperature of the air to the evaporation temperature.

Further, such evaporators need a great excess of air, owing to the unfavorable saturation conditions of the air, so that in addition to the great amount of heat required'the consumption of energy also is increased. On the other hand, evaporators have been proposed, in which the heat for the evaporation at reduced temperature is introduced by means of heated air. These systems have, compared to the space they require, a very small eificieney and are not economical because cf the large surface of liquid which is necessary.

My invention can be more readily, understood with reference to the accompanying drawing, wherein: t

Fig. l is a diagrammatic view of a device embodying the principle of my invention;

Fig. 2 is a chart wherein the operation cost is plotted against the evaporation temperatures, and

Figs. 3-5 are respectively schematic sections through three different embodiments of my invention.

It has now been discovered that evaporation can be carried out in a very economical manner and that also the drawbacks of the known evaporators can be avoided, if in a heated evaporator V of the kind diagrammatically represented in Figure 1 of the accompanying drawing an auxiliary gas is introduced thereinto and brought therein into-intensive contact with'the liquid to be evaporatedin order to reduce the partial pressure and therefore the evaporation temperature of the liquid and if additionally, the gas is conveyed before its admission to the heated evaporator, successively into a heat exchanger W1 for recovering the heat of the mixture of gas and vapor leaving the evaporator and into a. heat exchanger W2 for utilization of the waste heat of the heating-devices H and W3. The utilization of the incident or secondary heat given off from the evaporating system, while maintaining thermodynamic equilibrium between the gas and the vapor phase of the liquid is the main feature of the present process of evaporation, which renders the same more economical and more eflicient than known processes of this kind.

With an evaporation temperature situated below the temperature at which the gas enters the heat exchanger, the heat economy is still further 5 increased by the utilization of the heat supplied from outside. Although at lower temperatures the amount of power required for delivering the gas increases, evaporations for instance at C. are still considerably more economical than those 10 under vacuum.

Figure 2 shows a comparison of the optimum energy costs as a function of the evaporation temperature for, for instance, continuous evaporation of water with vapor as heating agent assuml5 ing equal unit costs for the amounts of energy. Curve A represents the cost units of energy for vacuum evaporation with utilization of condensation heat, and curve B that for known evaporators, with introduction of air at C. and with a relative humidity of 50 per cent, whereby the costs for the range of low temperatures which can only be employed in the new process, are indicated by a dotted section of said curve. Curve C shows the energy costs for the new process with partial pressure evaporation, while introducing air into the evaporator and recovering the heat of the mixture of air and vapor by way of direct heat exchange and that of the condensate by way of indirect heat exchange, andstill utilizing at 0 lower temperatures the heat of the difference of temperature. Curve D indicates the costs of the partial pressure evaporation with recovery of the heat of the air and vapor mixture by way of indirect heat exchange, extended also to the ranges of low evaporation temperatures. When using a gas of a relative. humidity below 50 per cent, the costs are even considerably more favorable.

Experiments have confirmed that, with the use of a partial pressure evaporation, with given conditions of operation and unit energy costs, savings exceeding 30 per cent. are obtained as compared with similar conditions in known evaporators. They have also shown that the evaporating efliciency, according to the nature of the heating agent used, is greater, in that it may amount to over 20 per cent. more than corresponds with the theoretical raising of the temperature gradient, between the temperature of the heating agent and the temperature of the evaporation point associated to the partial pressure in the evaporator system. This observation can be explained by the fact that no vapor bubbles can form on the heating surface as with the actual known evaporators, which would render the heating surface partially ineffective.

These advantages of partial pressure evaporation, when applied to muti-stage evaporators, which are per se very economical in respect of heat, produce still more favorable results.

evaporation temperatures for utilizing low pressure vapor or negative pressure vapor and the waste heat from furnaces, for increasing the evaporation efiiciency, and also for treating products which are temperature sensitive, but also for the reductionof the evaporation temperature of substances of high boiling point. This latter reduction of temperature which can at the same time be increased by the expedient of an alteration of pressure, combines the said technical advantages also with chemical advantages, such as checking decomposition, influencing chemical reactions, etc. Thereby new possibilities appear for carrying out, for instance, distillation of mixtures of high boiling point, such as those of the lubricating oil group, which are otherwise technically difllcult to control. Further, it is associated with a favorable heat recovery.

The partial" pressure evaporation can also be used advantageously above atmospheric pressure, as with the assumption of equal specific gas contents (quantity of gasto evaporated quantity of liquid) the absolute temperature gradient between the evaporation temperatures corresponding to the total pressure and to the partial pressure increasesv with higher pressure.

The new process is suitable for distillations and rectiflcations of liquid mixtures not only on account ofthe thermo-technical advantages, but also on account of the considerably smaller loss of solvents of lower boiling point as compared with similar conditions under vacuum, as according to the latter method considerable losses occur due to the great pressure difference and leakages in the apparatus resulting from the negative pressure.

For the evaporation and distillation of substances sensitive to oxygen there may be used with advantage as agents forthe reduction of the partial pressure, inert gases,such as waste gases from furnaces and industrial plants, nitrogen, hydrogen and the like;

The increase of evaporation is particularly advantageous for apparatus used in the chemical industry, as for these, in consideration of the purpose of application, the greatest possible heating surfaces with boiler volume are fixed and generally cannot be increased.

The new process is found to be particularly suitable for the evaporation of liquids which contain volatile, corrosive substances, such as acids and alkalies. For carrying off the acid or alkaline gas and vapor mixture, when using the vacuum evaporation method, expensive apparatus and machines, partially of corrosion-resistant materials, are necessary, whereas with the new process simple constructions of cheap material can be used. The evaporation can be conducted for instance with an excess of gas in accordance with Henry-Daltons law, in such a manner that the escaping acid or alkaline mixture of gas and vapor is unsaturated, thatis to say leaves the evaporator superheated, whereby the construction materials are less subjected to chemical attack.

A further advantage of the new process is that it is also suitable for the evaporation of foaming liquids also at low temperatures. By introducing the gas in condition of division or distribution over all the surface of the liquid, the foam p oduced on evaporation is immediately destroyed in statu nascendi. It is therefore possible to concentrate strongly foaming liquids, which in vacuo can not be evaporated at all, without great difli stance by surface heat exchange, or by means of direct heat transmission such as for instance by an intermixture of the heat yielding agent and the auxiliary gas. I

The mixture-of gas and vapor flowing oil from the evaporator system may be conducted to another suitable evaporator for still further utilization. Further, by partial'condensation the specific gas content can be increased, and this mixture, richer in gas, can be entirely or partly again utilized as agent for the reduction of the partial pressure by returning same in the circuit to the same evaporation system.

The liquid phase of the mixture of gas and vapor eliminated by partial condensation may be separated from the gaseous vapor phase by separators.

For the circulation of the auxiliary gas, there may be used, according to the pressure conditions, devices such as fans, compressors or the like.

The auxiliary gas may be introduced by means of distributors, for instance diffusers or the like over, on or under the level of the liquid, in such a manner that it is removed quickly in as saturated a state as possible from the surface of the liquid to be evaporated.

As agentfor the reduction of the partial pres sure there may be used gases or mixtures thereof with vapors, which in addition to the physical function are also, if necessary, capable of performing a chemical action, for instance hydro genization, oxydation and the like. small molecular weight have smaller specific gas contents but are not essentially more favorable in the heat and energy balance. On the other hand, such gases with greater heat conductivity such as for instance hydrogen, oifer special advantages in respect of heat transmission in the heat exchanger and condenser.

For the heating of the evaporator there may be used heating vapor of different pressure, particularly low pressure vapor, circulating heating liquids such as water, oil or the like, heating gases or furnaces supplied with oil, gas, coal or the like.

The invention will now be more fully described by reference to the accompanying drawing, but without the scope of theinvention being limited to the typical embodiments illustrated therein.

Figure 3 shows by a sectional view the arrangement of an open partial pressure evaporator according to the invention. I represents the evaporator, 2 is the heating body thereof provided with an entry 3 for the heating agent, I is the chamber in the evaporator for the mixture of gas and vapor and S the pipe for carrying oil! the mixture of gas and vapor. The auxiliary gas for the reduction of the partial pressure of the liquid II to be evaporated, for instance air, is supplied by the fan or compressor 6 to an ejectorlike suction device 1, which by way of a regulatable throttle valve 22 draws on at least a part of the air and. vapor mixture flowing oil! through the pipe 5. By way of mixing, a direct heat exchange takes place between the cold air Gases with and the warm mixture of air and vapor before mixture of air and vapor which is richer in air is conveyed by way of the collector Ill and pipe II into surface heat exchanger 23 in order to be heated, from which it is returned to the evaporator I where by means of the distributor I2 it is brought into intensive contact with the liquid in the evaporator, for instance below the liquid level therein. The remaining part of the mix ture of air and vapor flowing ofi through pipe 5 escapes by way of the discharge opening I3 to the atmosphere or can be further utilized for other purposes, as for instance in the multi-stage evaporator according to Figure 5.

The heating of the evaporator is effected in this arrangement by means of vapor. The water of condensation of the heating body 2 is de liver-ed through the discharge pipe It to the surface heat exchanger 23 where it gives off its heat to the air to be introduced into the evaporator. By this means, the vapor heat liberated by the relief of pressure, as well as the heat of the condensate can be utilized up to the temperature of the entering air. The cooled condensate flows off from the heat exchanger 23 by way of the discharge pipe 24. By heating the air to be introduced, the same is enabled to again take up humidity.

When operation takes place within such ranges of temperature, for which the air introduced through the throttle valve 20 has a higher temperature than the evaporation temperature of the liquid I4, it is advantageous to work with closed throttle valve 22. Under these conditions, only the waste heat of the heating agent is utilized in the heat exchanger 23 for the heating of the air to be introduced.

Figure 4 shows a closed partial pressure evaporator with circulation of the auxiliary gas. This form of construction is particularly suitable for evaporation and distillation of liquids, such as liquids of a low boiling point. The arrangement as represented is particularly suitable for fractionating by rectification and dephlegmation of various liquid mixtures, also those of azeotropic kind. Thus it is possible to produce also technically anyhydrous alcohol by fractionating under partial pressure. As is well-known, at low temperatures the azeotropic point is displaced into the range of higher alcohol contents. The new process, as compared with the rectification in vacuo, is very economical, as the alcohol losses are reduced to a The system is also particularly suitable for mixtures of high boiling point, which hitherto have been treated in vacuo according to known processes for instance fractionating of lubricating oils, etheric oils, tars for distillation, etc.

The method according to Fig. 4 is realized at any pressure in such a way that the mixture of gas and vapor flowing from the body of the evaporator or still I passes into a column system 36 where a thorough mixing and also a heat exchange of the mixture of gas and vapor and reflux takes place, whereby the vapor composition of the mixture of gas and vapor changes in such a manner that the vapor component is strengthened or enriched. This latter mixture flows through a dephlegmator 31, in which by heat transmission to the gas returned into the circuit, a part of the distillation vapors is precipitated by cooling and conducted as reflux into the column 36, whereas the mixture of gas and vapor which is richer in gas is cooled in the condenser 38. By separation in the cyclone 3, the distillate is removed by way of the discharge pipe 3 and the gas is conducted by way of the dephlegmator 31 and the delivery device 6 back to the body of the still where it is brought through the distribution I2 into intensive contact with the liquid contained in the still. If a larger re- 5 flux is necessary than corresponds to the heat exhange in the dephlegmator 31, such reflux will be obtained by a further cooling in known manner of the mixture of gas and vapor flowing oi! from the dephlegmator 31.

The heating of the evaporator I is effected with steam by means of the heating body 2. The heat of the condensate in said body 2 can be utilized for evaporation in such a manner that the condensation water is conducted from the heating body 2 by way of a discharge member I3 to a further heating body I I, whereby the vapor heat liberated by the relief of pressure, as also the heat of the condensate, can be utilized up to evaporation temperature. The intermediate heating body I I, for the sake of better utilization of the heat of the condensate caused by the low hydrostatic level of the liquid in the evaporator, is to be arranged above the main heating body 2.

In case of distillation of liquids with high boil- 25- ing point, such as lubricating oils or the like, the heating of the evaporator I is preferably produced by means of a furnace with gas, oil or coal sup- P y- In all cases, the utilization of the waste heat 30 of the heating device of the evaporator can take place, as was described in regard to Figure 3, by heating the gas to be introduced into the evaporator in a heat exchanger like that 23 of Figure 3 inserted into the air admission pipe leading to the 35 evaporator.

The methods of carrying out the process described according to Figures 3 and 4 may also be applied with multi-stage evaporators as final stages after the pressure stages, in such a manner that for heating the final stages the vapors of the previous stages are utilized.

Figure 5 represents an application of the process with multi-stage evaporators with, for example, three partial pressure stages. This form of construction is suitable for concentrating strong solutions or for separating salts from saturated solutions, as by the known rise of the evaporation point with increasing concentration, a greater temperature gradient is necessary. By a cascade arrangement, the system can be used with advantage for continuous evaporation. Particularly with a heating with low pressure vapor, it is found to be very economical.

The arrangement is conceived for open partial pressure evaporation with three stages a, b, 0.

Stage a with lowest evaporation temperature and, therefore, with highest specific air content corresponds, to a system of indirect heating of the air by the outflowing mixture of air and vapor. The air is conducted by way of the throttle valve 20 and the fan 6, to a surface heat exchanger 25 and by way of the distributor I2 to the evaporator I where it comes in intensive contact with the liquid. From the mixture of air and vapor flowing off, by indirect heat emission to the air, a part of the vapor is condensed and is carried off as condensation liquid by way of the discharge pipe 26, whereas the mixture of air and vapor which is poorer in vapor is thereafter heated in the heat exchanger II by the heat of the condensate from stage b and passes then through the distributor into the evaporator of stage b. In the evaporator of stage D, owing to evaporation, the specific air content decreases.

and the evaporation temperature 01' the solution is correspondingly raised. Similarly the mixture is delivered by way of the heat exchanger 44 to the evaporator of stage 0, where the evaporation point is still further raised. If, for instance, the dilute solution to be concentrated is allowed to flow. continually over from stage 0 by way of stage I; to stage a, it is possible to maintain a prescribed temperature gradient between the heating agent and the solution and therefore a definite performance in every stage in spite of great rise of the boiling point of the solution which is concentrated from stage to stage. It is also possible to cause the liquid to be evaporated to move in the opposite direction, which is advantageously the case, for instance, with solutions which are temperature-sensitive in the initial concentration, but in that case the evaporation output is reduced.

In order to be able to maintain the most favorable utilization of heat for a definite evaporation point, there should be provided regulation of the evaporation temperature by alteration of the quantity of the auxiliary gas to be introduced into the evaporator, or influencing the cooling action of the condenser or a combination of both measures. In order to maintain the optimum economy, the two throttle valves 20 and 22 in Figure 3 may for instance be adjusted by a suitable regulating device, in such a manner that the specific gas content and, therefore, the evaporation temperature itself on variation of the temperature and the relative humidity of the auxiliary gas, and also of the heating medium temperature, is always maintained constant. Such regulation, particularly at low evaporation temperatures in whose regions the energy expenditure curve falls or rises very steeply, exhibits great advantages and is, therefore, indispensable for economic operation.

What I claim is:

1. A process for evaporating liquids, especially temperature-sensitive liquids in a heated evaporator supplied with an auxiliary gas, consisting in bringing said gas within the evaporator into intensive cont-act with the liquid therein with consequent reduction of the partial pressure of the liquid and of its evaporation temperature, then evacuating said gas with produced vapor in mixture from the evaporator and transferring by way of heat exchange, for recovery, the heat of the heating system of the evaporator to the gas to be introduced into the evaporator.

2 A process for evaporating liquids, especially temperature-sensitive liquids, in a heated evaporator supplied with an auxiliary gas, consisting in bringing said gas within the evaporator into intensive contact with the liquid therein with consequent reduction of the partial pressure of the liquid and of its evaporation temperature, then evacuating said gas with produced vapor in mixture from the evaporator and transferring by way of heat exchange, for recovery, both the heat of the evacuated gas and vapor mixture and the waste heat of the heating system of the evaporator to the gas to be introduced into the evaporator.

' 3. A process for evaporating liquids, especially temperature-sensitive liquids, in a heated evaporator supplied with an auxiliary gas, consisting in bringing said gas within the evaporator into intensive contact with the liquid therein with consequent reduction of the partial pressure of the liquid and of its evaporation temperature,

then evacuating said gas with produced vapor in mixture from the evaporator, transferring by way 01' heat exchange, for recovery, the heat 01 the evacuated gas and vapor mixture to the gas to be introduced into the evaporator, causing separation of at least one part of the gas from the gas and vapor mixture and returning same in circuit back to the evaporator.

4. A process for evaporating liquids, especially temperature-sensitive liquids in a heated evaporator supplied with an auxiliary gas, consisting in bringing said gas within the evaporator into intensive contact with the liquid, therein with way of heat exchange, for recovery, both the heat of the evacuated gas and vapor mixture and the waste heat of the heating system of the evaporator to the gas to be introduced into the evaporator, causing separation 01' at least one part of the gas from the gas and vapor mixture and returning same in circuit back to the evaporator.

5. A process for evaporating liquids, especially temperature-sensitive liquids, in a heated evaporator supplied with an auxiliary gas, consisting in bringing said gas within the evaporator into intensive contact with the liquid therein with consequent reduction of the partial pressure of the evaporator, transferring by way of heat exchange, for recovery, the waste heat of the heating system of the evaporator to the gas to be introduced into the evaporator and utilizing also the heat contained in the auxiliary gas for the evaporation in the evaporator.

6. A process for evaporating liquids, especially temperature-sensitive liquids, in a heated evaporator supplied with an auxiliary gas, consisting in bringing said gas within the evaporator into intensive contact with the liquid therein with consequent reduction of the partial pressure of the liquid and of its evaporation temperature, then evacuating said gas with produced vapor in mixture in unsaturated condition from the evaporator, transferring by way of heat exchange, for recovery, the waste heat of the heating system of the evaporator to the gas to be introduced into the evaporator and utilizing also the heat contained in the auxiliary gas for the evaporation in the evaporator.

7. A process for evaporating liquids, especially temperature-sensitive liquids, in a heated evaporator supplied with an auxiliary gas, consisting in bringing said gas within the evaporator into intensive contact with the liquid therein with consequent reduction of the partial pressure of the liquid and of its evaporation temperature, then evacuating said gas with produced vapor in mixture in unsaturated condition from the evaporator, transferring by way of heat exchange, for recovery, the waste heat of the heating system of the evaporator to the gas to be introduced into the evaporator and utilizing also the heat contained in the auxiliary gas for evaporation in the evaporator, evacuating gas from the gas and vapor mixture, and conveying the evacuated gas into at least one further evaporator of the system for utilizing same therein.

CASIMIR HEILER. 

